An office worker identified only as Bill was quoted as saying he heard gunfire and saw scores of people running away in terror.
"It was sheer panic," he said. "I've never seen anything like it."
Seattle police said on Twitter that detectives were investigating after "six people were wounded by gunfire, one fatally".
"The suspect(s) fled the scene," police added.
A Fire Department spokesman described the fatality as a woman about 40 to 50 years of age, who was found dead at the scene.
Seven victims had been brought to Harborview Medical Center's emergency room, a hospital spokeswoman said.
The police department said officers and medics were providing aid to victims at the scene.
KOMO-TV said a 9-year-old boy was among the wounded.